1. Field of the Invention
This invention relates to non-volatile memory devices and is more particularly concerned with certain apparatus and methods based on new concepts of memory state demarcation and programming reference signal generation for multi-bit electrically alterable non-volatile memory (EANVM) cells.
2. Related Background Art
In conventional single-bit per cell memory devices, the memory cell assumes one of two information storage states, either an "on" state or an "off" state. This combination of either "on" or "off" defines one bit of information. A memory device using such single-bit cells to store n bits of data (n being an integer greater than 0) thus requires n separate memory cells.
Increasing the number of bits which can be stored in a single-bit per cell memory device involves increasing the number of memory cells on a one-for-one basis with the number of bits of data to be stored. Methods for increasing the number of memory cells in a single memory device have relied upon advanced manufacturing techniques that produce larger chips containing more memory cells or that produce smaller memory cells (e.g., by high resolution lithography) to allow more memory cells to be placed in a given area on a single chip.
An alternative to the single-bit per cell approach involves storing multiple bits of data in a single memory cell. Previous approaches to implementing multiple-bit per cell non-volatile memory devices have typically involved mask-programmable read only memories (ROMs). In one of these approaches, the channel width and/or length of the memory cell is varied such that 2.sup.n different conductivity values are obtained which correspond to 2.sup.n different states, whereby n bits of data can be stored by a single memory cell. In another approach, the ion implant for the threshold voltage is varied such that the memory cell will have 2.sup.n different voltage thresholds (Vt) corresponding to 2.sup.n different conductivity levels corresponding to 2.sup.n different states, whereby n bits of data can be stored by a single memory cell. Examples of memory devices of these types are described in U.S. Pat. No. 4,192,014 to Craycraft, U.S. Pat. No. 4,586,163 to Koike, U.S. Pat. No. 4,287,570 to Stark, U.S. Pat. No. 4,327,424 to Wu, and U.S. Pat. No. 4,847,808 to Kobatake.
Electrically alterable non-volatile memory (EANVM) devices capable of storing multiple bits of data per cell are also known. In these devices, the multiple memory states of the cell are demarcated by predetermined reference signal levels that define boundaries between adjacent memory states. The memory cell is read out by comparing a signal from the cell with the reference signals to determine the relative levels of the cell signal and the reference signals. The comparison results indicate whether the cell signal level is above or below the respective memory state boundaries, and thus collectively indicate the programmed state of the cell corresponding to the stored data. The comparison results are encoded to reproduce the stored data and complete the cell readout operation. Generally speaking, the number of reference levels required to demarcate n memory states for storing n bits of data is 2.sup.n -1. The number may be greater if, for example, the uppermost or lowermost memory state is to be bounded on both sides.
Previous approaches to programming multi-bit EANVM cells are based on a repeated cycle of programming and readout of the cell. The cell is programmed incrementally, by the application of programming pulses, and the programmed status of the cell is checked repeatedly during the programming process by reading out the memory state of the cell as described above to verify the attained level of programming. Programming is continued until the target memory state has been reached, as indicated by the readout of the cell.
In order to minimize the possibility of readout errors, the programming level of a multi-bit EANVM cell should be set with a margin relative to the reference signal level or levels that demarcate the target memory state. The programming margin should be sufficient to avoid readout errors that might occur due to variations in operating characteristics of the cell with changing conditions such as temperature, system voltages, or mere passage of time. More particularly, if the cell is programmed too close to a memory state boundary, slight variations in the operating characteristics could shift the cell signal level relative to the state boundary level, resulting in an error upon subsequent readout of the cell.
Program margining is not particularly problematical in single-bit per cell memory devices, since there are only two memory states, and thus no intermediate memory states. Because it is impossible to overshoot the target state by overprogramming the cell, the cell may simply be programmed to set the cell signal level as far as possible from the reference level bounding the two memory states.
By contrast, the presence of one or more intermediate memory states makes program margining a significant concern in the case of multi-bit per cell devices, because an intermediate memory state requires a programming margin that provides adequate separation from two boundary levels--that is, the boundaries of the intermediate memory state with both the state above and the state below. Programming the cell too close to either level can result in a readout error. Also, both overprogramming and under programming must be avoided to prevent overshooting and undershooting the target intermediate state.
Previous program margining techniques include techniques that, for programming purposes, shift the cell signal level or the reference signal levels relative to their values during normal memory readout. The effect in either case is that, for a given programming amount of the cell, the cell will read differently during programming than during a normal readout operation. The difference corresponds to the shift amount of cell signal or the reference signals and provides a programming margin. Examples of these techniques are found in U.S. Pat. No. 5,172,338 to Mehrotra et al. and in Beliker et al., "A Four-State EEPROM Using Floating-Gate Memory Cells," IEEE Journal of Solid State Circuits, Vol. SC-22, No. 3, June 1987, pp. 460-463.
Another margining technique involves the provision of additional reference signals having levels intermediate those of the state-demarcating reference levels. The intermediate reference levels define program margin ranges in conjunction with the state-demarcating levels. After the cell reaches the target memory state, as indicated by comparison with the state-demarcating signals, programming is continued based on further comparison of the cell signal with one or more intermediate reference signals to provide a programming margin. An example of this technique is found in U.S. Pat. No. 4,964,079 to Devin.
In the above-described approaches to programming multi-bit per cell EANVM devices, the programming speed (total time to program a cell to a target state) is substantially limited by the need for repeated readout of the memory cell during the programming process. Also, the aforementioned program margining techniques impose substantial complications on the overall circuit design due to the need to shift the cell signal level or the state-demarcating reference signal levels, or to provide intermediate reference levels for establishing program margin ranges in conjunction with the state-demarcating reference signal levels. Furthermore, these margining techniques do not assure an optimum programming margin throughout variations in operating characteristics of the cell, because they do not precisely track such variations with changing conditions that affect the operating characteristics.